State-of-the-art electric energy conversion relies on a three-phase power is network with alternating currents (AC) at 50 Hz or 60 Hz frequency and voltage levels ranging from several hundreds of Volts to hundreds of thousands of Volts. The conversion of rotating mechanical energy into electric energy and vice versa is done by generators and by motors, respectively. Those rotating machines can be divided into asynchronous and synchronous apparatuses.
Motors and generators comprise a stator and a rotor. The rotor of the machine rotates inside the stator bore of the stator. Rotating machines generate the magnetic field typically through rotor pole windings. The number of rotor poles and the frequency of the stator magnetic field define the number of revolutions per minutes (rpm) of the rotating machine.
As known, the stator-bars of generators are generally made of a top and a bottom layer. The bars of top and bottoms layer are connected to each other by copper pieces called lugs, such to form electrical coils. An electrical connection is then needed between the electrical coils and the terminals of the generator, which lead the power to the outside through the generator casing. For this purpose, this connection is established by copper bars (of rectangular or round cross-sections) bent around the support structure on its outermost diameter.
Generally, the first and the last bar of a coil of the winding are connected to the terminals. These bars (usually six per layer, equally distributed over the circumference) are called phase bars and are specially designed to lead the generated electrical power of the generator into an external grid. The connection between the phase bars and the terminals, which lead the power through the generator's casing and are called round connections or phase rings.
They are electrically insulated against each other, and quite sophisticated shapes are required for reaching the phase bars and connecting them to the generator terminals whilst maintaining the required insulation distances.
The round connections are generally fixed to the support structure to prevent high vibrations.
The end winding support structure of the generator is located at the front of the press plates and the outer side of the end winding of the stator bars. This support structure is used for the fixation of the stator bars ends, to apply mechanical strength to the end winding and withstand the forces occurring during normal operation and accidental incidents. The support usually consists of brackets mounted in axial direction and often of supporting rings to stiffen the structure.
The ends of the round connections, which are foreseen to be connected to the phase bars, sometimes cannot be fixed to the supporting structure over a long distance, since the position of the phase bars might be located between the supporting brackets.
As a result of this, round connection ends have a high risk of vibrations.
As a consequence, an important problem is the vibration of round connection arms to the phase lugs. The Eigen frequency of these arms often need to be detuned (e.g. no resonance between 100 and 140 Hz for 60 Hz applications) or at least the vibration levels have to be reduced to a minimum.
Cracks in the strands close to the phase lugs and in the lugs can be found in several generators. These cracks are mostly caused by high vibration levels of the round connections arms. Some round connections arms have Eigen frequencies close to 120 Hz (for 60 Hz applications) or 100 Hz (for 50 Hz applications).
Up to date, some existing solutions suggest applying a very stiff support between the round connection arm and the support ring with cords or tape together with resin. This solution detunes the Eigen-frequency out of the range of 100 to 140 Hz very well and reduces the amplitude of the vibrations.
On the other hand thermal expansion, which does occur during operation of the machine, should not be blocked. Such thermal expansion mainly occurs along the axial direction (usually causing a displacement of the connection arm of about 2-3 mm), but also in radial and tangential directions. Therefore a “stiff” solution is not possible.
The stiff solution hinders the thermal expansion, which increases the stress at the strands close to the lug and considerably increases the danger of cracks in the strand. On the other hand, the high stress to the cords/tapes of the stiff support may lead to a loosening of the connection over time, which renders this support useless for reducing the vibrations.
Generally, the stiff support according to the teachings of the prior art entails a high risk of breakage caused by the blocking of the thermal expansion. The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties.